It is the part where Ragina is sitting with Aaron and another guy across from her. Ragina states that she wants to lose weight. The guy sitting across from her says, "you're crazy". You can hear him say that, before his mouth even moves.

Aron is the one that says, "You're crazy." You can tell by his voice. The other guy is moving his lips because he is talking to his own friends. The makers of the movie wanted to make a realistic cafeteria scene, and not only one single person talks at a time.

If you even looked.. you'd see there's no one sitting across from them. That was Aaron talking. When Cady jumps over the table there's no one sitting there. And they show you about 5 other shots of them sitting there.. And in all no one is sitting there.